The invention relates to a network vector analyzer for the millimeter range. Such an analyzer comprises a millimeter wave source, a first oscillator, with a transmitting and a microwave receiving side. For obtaining measurement values, a part of the sender signal is coupled out of the wave guide to the transmitting antenna and is converted down to provide a measurement signal for a vector voltage meter. The signals are high-frequency output signals in the mm wave length range which are converted down in a phase-and-amplitude stabilized manner to frequencies suitable for further processing. The local oscillations are removed from the reference oscillator during down conversion.
The main problem in the network analysis in the millimeter wave area is a suitable millimeter wave source. With the use of a narrow band heterodyne receiver, it is possible to determine signals up to several dB of the physical noise limit (Pnoise=kTB, with B=1 Hz and T=273K, these are 174 dBm). Consequently, the lower detection limit depends mainly on the bandwidth of the receiver filter. The maximum power that can still be obtained with this frequency-selective filter from the source depends on the spectral power density contained in the receiver band width and the frequency stability thereof.
The basic concept is described in U.S. Pat. No. 5,119,035. The millimeter frequency generated by multiplication and the xe2x80x9cpumping signalsxe2x80x9d required for the down mixing of the receiver signal are derived from two 8-18 GHz oscillators. In order to obtain a phase relationship between the two YIG controlled transistor oscillators, which is needed for the vector measurement, both are coupled, by way of a phase control circuit, to a reference source in a phase-rigid manner. Like in a scalar setup, the achievable measurement range is limited by the efficiency of the multipliers used. It is indicated to be 70-80 dB in the D band.
A further development of this concept relates to a Gunn-oscillator, which is phase stabilized by way of the supply voltage. This arrangement, however, has a weakness. The used semiconductor oscillators provide the output power of several 10 mW, but their electrically tunable frequency range is limited by the required mechanical resonance space. For example, with a Gunn-oscillator having a mean frequency of 144 GHz, this frequency range is only 5 GHz. Furthermore, such oscillators, which are based on indium phosphate, are commercially available with reasonable power outputs only up to 150 GHz. The use of an independent backward wave oscillator (BWO) cannot be taken into consideration because of its insufficient frequency resolution.
It is the object of the present invention to provide a network vector analyzer, which is highly dynamic in the range of 70-190 GHz with commercially available components by which a network can be examined under the effect of millimeter waves over a large measuring range or, respectively, a large wavelength range.
In a vector network analyzer with a transmitting and a receiving side, each including a super heterodyne receiver which are downward mixing, a high voltage power supply for providing an accelerating voltage is arranged in series with a second power supply with a controllable lower output voltage so that unavoidable voltage variations in the acceleration voltage can be controlled. The second power supply is connected to the output of a phase and frequency discriminator by way of an amplifier and a low-pass filter. The discriminator has two inputs, one receiving the downwardly mixed transmitting side signal and the other the signal provided by a reference oscillator having a constant frequency. As millimeter wave oscillator a backward wave oscillator is used whose frequency drifting by temperature changes or by aging is compensated by a control signal for the second power supply, which control signal is derived from the phase and frequency discriminator.
The core of the network analyzer is a phase control circuit, which connects the first millimeter wave source to a third frequency-stable reference source. In order to facilitate an accurate frequency adjustment and to obtain for the transmitting signal a clean spectrum, the oscillator should have a quality corresponding at least to that of quartz oscillators. Better oscillators such as a rubidium-time-standard may be used, if a 10 MHz signal can be obtained therefrom. If a quartz oscillator is used as a reference source, the quartz reference forms the reference signal.
The first millimeter wave source is a transit time tube, which determines the transmission bandwidth based on its parameters. Of the group of the useable transit time tubes, the backward wave oscillator (BWO) has proved to be suitable wherein the oscillation frequency is controlled by way of the acceleration voltage.
The network apparatus required herefor have a large proportion of interference voltages such as network humming and remainders of the voltage control, which directly affect the spectrum of the output signal. Even if the acceleration voltage would be free of humming, a thermal expansion of the delaying structure would lead for the first oscillator, which is in the form of a tube, to a drifting of the frequency. In order to compensate for these interferences, a small independent DC source whose amplitude can be rapidly controlled is arranged in series with the power supply unit. The control value is formed from the downwardly mixed partial signal of the transmitter signal by phase comparison with the reference signal from the reference oscillator wherein a low pass and an amplifier are disposed in between. With an appropriate adjustment of the control parameters, a spectral width of several megahertz bandwidth is controlled down to less than 100 Hz. In this way, a receiver with a small receiving bandwidth and a vector network channel analyzer with high dynamics is realized.
The reference oscillator with constant frequency times the microwave oscillator or the microwave oscillators, which form the microwave oscillator for the downward mixing or generally the steps of downward mixing. The respective downward steps in the transmitting and receiving-side super-heterodyne-receiver may be the same or different. Important is only that, at the end of the downward mixing, the frequency of the constant-frequency reference oscillator is reached.
Important for the inductive disturbances, inspite of different first intermediate frequency levels, is the spatial separation of the two superheterodyne receivers since, with the same multiple downward steps, both include further intermediate frequency steps of the same frequency. Also, with a separate development of partially identical local oscillator frequencies in both multi-step superheterodyne receivers, the occurrence of direct cross-coupling cannot be prevented. With identical intermediate frequencies, it is to be taken into consideration that the transmitting and receiving side first mixers are operated by way of an amplifier with high isolation, which is the same as a high backward attenuation. The isolation normally corresponds at least to the amplification. At the same time, these amplifiers must not have such a large bandwidth that the first intermediate frequency (in the example 640 MHz) is noticeably amplified.
In order to achieve high dynamics with, at the same time, a high sensitivity, the main amplification should be concentrated on the lowest intermediate frequency. This however would have the disadvantage that also the unavoidable inductive interference of the reference frequency would be highly amplified. This would result in a substantial limitation of the dynamics.
The advantages of a vector network channel analyzer reside in the fact that a complete electronic control can be established whereby mechanical resonators are eliminated. Frequency drifting of the backward wave tube by temperature variations and/or aging are compensated by the phase- and frequency discriminator and the control signal derived therefrom. Using fewer backward wave oscillatorsxe2x80x94in contrast to semiconductor oscillators such as Gunn diodesxe2x80x94the complete mm-wave range is to be covered. Already with a D-band based wave oscillator, half of the W-, the complete D- and half of the G-band is covered. The circuit concept can be utilized in principle, with all available backward wave oscillators. Since no special development of specific high frequency building components is necessary and the needs can be satisfied by commercially available components, a network channel analyzer having good properties can be made at an economical price.
An embodiment of the invention will be described below on the basis of the accompanying drawings.